1. Field of the Invention
The present invention relates to a display device including an organic light emitting element. In particular, the present invention relates to a display device (an organic EL display) including a light emitting element having an organic compound (an organic light emitting element) which emits light by being applied with electronic field. Moreover, the present invention relates to an electronic appliance using such a display device for a display portion.
2. Description of the Related Art
An organic light emitting element has a structure in which a layer containing an organic compound (an organic light emitting material) which generates electroluminescence by being applied with electric field (hereinafter, referred to as an organic compound layer), is interposed between a pair of electrodes (a first electrode and a second electrode). The organic light emitting material is classified into a material which can convert energy upon returning to a ground state from a singlet excited state into light emission (a fluorescent material) and a material which can convert energy upon returning to a ground state from a triplet excited state into light emission (a phosphorescent material).
A display device including such an organic light emitting element (an organic EL display) has characteristics of high response speed suitable for movie display, requiring low driving voltage, and consuming less power, and therefore, the display device has been attracting attention as a next generation display for a cellular phone, a portable information terminal, or the like.
Differing from a liquid crystal display device, the organic EL display emits light by itself, and therefore, it has a feature of good viewing angle. Therefore, the organic EL display is more suitable as a display used in the open air than a liquid crystal display so that various uses of the organic EL display have been proposed.
An organic light emitting element which is formed over a light transmitting substrate and emits light from both sides of the light transmitting substrate (hereinafter, referred to as a dual emission type organic light emitting element) has been known (e.g., see patent document 1). This dual emission type light emitting element has a feature of emitting light toward the both sides of the substrate, and therefore, there has been an expectation of widespread application of the dual emission type light emitting element by utilizing such a feature.
[Patent Document 1]: Japanese Patent Application Laid-Open No. 2004-265691
Further, the patent document 2 discloses a display device which displays full color images by attaching panels including dual emission type organic light emitting elements which emits different colors of light to each other such that light emitting regions of the organic light emitting elements provided over each panel are not overlapped with each other as seen from a viewer side.
[Patent Document 2]: Japanese Patent Application Laid-Open No. 2005-71693
An object of the present invention is to provide a display device which utilizes the above mentioned features of a dual emission type organic light emitting element. Another object of the present invention is to improve image quality of an organic EL display by utilizing a dual emission type organic light emitting element. In order to improve the image quality, concretely, the following three problems can be given.
First, a problem of controlling brightness of a displayed image can be given. Suitable brightness of an organic EL display device is differed in a case of using the organic EL display device in a bright-light location such as an outdoor location and in a case of using it in doors. Therefore, it is necessary to control brightness of the display device depending on brightness of a location of use.
Next, a problem of improving chromaticity can be given. For example, when color display with red (R), green (G), and blue (B) is performed, only colors within a triangle of a chromaticity coordinate of red (R), a chromaticity coordinate of green (G), and a chromaticity coordinate of blue (B) in a chromaticity diagram, have been expressed, and therefore, there has been a limitation in a range of colors which can be expressed. If the range of colors which can be expressed, can be more widened, the number of colors to be expressed can be increased so that more microscopic and realistic images can be displayed.
Furthermore, a problem of balance of colors can be given. It has been known that visibility is differed depending on colors. The visibility indicates sensitivity of the eye to light. A yellow-green wavelength in the vicinity of 555 nm shows highest visibility. As being shifted to shorter wavelengths than 555 nm, the visibility is reduced. Meanwhile, as being shifted to longer wavelengths than 555 nm, the visibility is reduced. Accordingly, the visibility of green light is higher than the visibility of red light and the visibility of blue light. Thus, it is necessary to balance the colors depending on the visibility. Furthermore, there is also a problem that a blue organic light emitting element has lower luminance than red and green organic light emitting elements, and therefore, it is also necessary to balance the luminance of red, green, and blue light.
The present invention is made in view of the above described problems.
In an aspect of the present invention, a display device includes a first substrate over which an organic light emitting elements is provided, and a second substrate over which an organic light emitting element is provided, wherein the first substrate and second substrate are provided to face each other, at least either the organic light emitting element provided over the first substrate or the organic light emitting element provided over the second substrate emits light toward both surfaces of the substrate, and a light emitting region of the organic light emitting element provided over the first substrate and a light emitting region of the organic light emitting element provided over the second substrate are placed such that they are at least partly overlapped with each other as seen from a viewer side.
In order to form an organic light emitting element which emits light toward both sides of a substrate over which the organic light emitting element is provided, a first electrode and a second electrode of the organic light emitting element may be formed using transparent conductive films. Accordingly, in the above described structure, the phrase “at least either the organic light emitting element provided over the first substrate or the organic light emitting element provided over the second substrate emits light toward both surfaces of the substrate” indicates that a first electrode and a second electrode of at least either the organic light emitting element provided over the first substrate or the organic light emitting element provided over the second substrate are formed using a transparent conductive film.
Note that, the first electrode of the organic light emitting element is placed closer to the substrate over which the organic light emitting element is provided than the second electrode of the organic light emitting element. Thus, in the pair of electrodes of each organic light emitting element provided over the first substrate, the electrode being closer to the first substrate is the first electrode.
In the above mentioned structure, a first organic light emitting element is provided over the first substrate, a second organic light emitting element, a third organic light emitting element, and a fourth organic light emitting element are provided over the second substrate, and one pixel includes the first to fourth organic light emitting elements.
In the above mentioned structure of the present invention, a first organic light emitting element is provided over the second substrate, a second organic light emitting element, a third organic light emitting element, and a fourth organic light emitting element are provided over the first substrate, and one pixel includes the first to fourth organic light emitting elements.
In the above mentioned structure of the present invention, a first organic light emitting element and a second organic light emitting element are provided over the first substrate, a third organic light emitting element and a fourth organic light emitting element are provided over the second substrate, and one pixel includes the first to fourth organic light emitting elements.
Further, in the present invention, the first to fourth organic light emitting elements emit different colors of light from one another.
In the above mentioned structure of the present invention, a first organic light emitting element, a second organic light emitting element, and a third organic light emitting element are provided over the first substrate, a fourth organic light emitting element, a fifth organic light emitting element, and a sixth organic light emitting element are provided over the second substrate, and one pixel includes the first to sixth organic light emitting elements.
Further, in the present invention, the first to third organic light emitting elements emit different colors of light form one another.
Furthermore, the first to sixth organic light emitting elements emit different colors of light form one another.
That is, in the above mentioned structure of the present invention, two colored or three colored organic light emitting elements selected from four to six colored organic light emitting element of red, green, and blue organic light emitting elements and one to three colored organic light emitting elements selected from red, green, blue, a complementary color of red, a complementary color of green, a complementary color of blue, and white, are provided over the first substrate, and the rest of the four to six colored organic light emitting elements is/are provided over the second substrate.
Further, various kinds of combinations of organic light emitting elements provided over the first substrate and the second substrate can be given. In relation to the combination of organic light emitting elements provided over the first substrate and the second substrate, the following first to fourth structures of the present invention will be described in more detail.
In the first structure of the present invention, red, green, blue, and while organic light emitting elements are provided over a first substrate or a second substrate. That is, one pixel includes the four colored, i.e., red, green, blue, and while organic light emitting elements.
Note that, in the present specification, a minimum unit required for displaying an image is referred to as a pixel, and the pixel includes a plurality of dots. For example, when an image is displayed using three colors of red, green, and blue, one pixel is constituted by three different colored dots, i.e., a red dot, a green dot, and a blue dot as one set.
Therefore, it can be said that in the first structure of the present invention, one pixel includes a red dot, a green dot, a blue dot, and a white dot.
As the first structure of the present invention, for example, an example in which red, green, and blue organic light emitting elements are provided over one of the first substrate and the second substrate, whereas a while organic light emitting element is provided over the other substrate, can be given.
Providing the white organic light emitting element over the substrate different from the substrate over which the red, green, and blue organic light emitting elements are provided makes it possible to improve brightness of an entire display screen of a display device.
When an organic EL display is used as a display portion of an electronic appliance used in the open air and in doors regardless of location such as a cellular phone and a portable information terminal, if brightness of a display screen is not suitably controlled in accordance with surrounding brightness, visibility is degraded. In this case, by increasing luminance of only the white organic light emitting element provided over the substrate different from the other substrate over which the red, green, and blue organic light emitting elements are provided, the brightness of only the display screen can be improved.
Since the substrate having only the white organic light emitting element is provided separately from the substrate having the red, green, and blue organic light emitting elements for performing color display, the luminance of the while organic light emitting element can be changed independent from the red, green, and blue organic light emitting elements. Accordingly, the brightness of the display screen can be easily controlled.
Further, when white color is displayed on a conventional organic EL display which can perform color display, red, green, and blue organic light emitting elements emit light and three colors of light emitted from the organic light emitting elements are mixed to be while light. However, in the structure of the present invention in which a substrate having red, green, and blue organic light emitting elements and a substrate having a white organic light emitting element are overlapped with each other, only the white organic light emitting element emits light while the red, green, and blue organic light emitting elements emit no light so as to display white color. Therefore, power consumption can be more reduced as compared to the case of displaying white color by making the red, green, and blue organic light emitting elements emit light.
Note that, the arrangement of the red, green, blue, and while organic light emitting elements may, for example, be either a case A or a case B as follows.
The case A: one dot of a white organic light emitting element is provided over a second substrate such that it faces a region where one dot of a red organic light emitting element, one dot of a green organic light emitting element, and one dot of a blue organic light emitting element are provided over a first substrate.
The case B: dots of white organic light emitting elements are respectively provided at positions over a second substrate corresponding to regions over the first substrate where a dot of a red organic light emitting element, a dot of a green organic light emitting element, and a dot of a blue organic light emitting element are provided over the first substrate. Specifically, one dot of the white organic light emitting element is provided over the second substrate such that it faces the one dot of the red organic light emitting element provided over the first substrate, another one dot of the white organic light emitting element is provided over the second substrate such that it faces the one dot of the green organic light emitting element provided over the first substrate, and another one dot of the white organic light emitting element is provided over the second substrate such that it faces the one dot of the blue organic light emitting element provided over the first substrate.
In the case B, it is possible to independently control luminance of the white organic light emitting elements, which are provided at the positions corresponding to each light emitting region of the red, green, and blue organic light emitting elements provided over the first substrate. Therefore, red, green, and blue colors can be respectively controlled. That is, for example, when only the white organic light emitting element provided at the position corresponding to the red organic light emitting element emits light, the white emission and red emission is mixed to exhibit pink emission while green emission and blue emission can be exhibited as they are, and hence, reproducibility of colors can be improved.
An example of providing the red, green, and blue organic light emitting elements over the first substrate while providing the white organic light emitting element(s) over the second substrate are described in each of the cases A and B. Alternatively, the red, green, and blue organic light emitting elements provided over the first substrate and the white organic light emitting element(s) provided over the second substrate can be counterchanged. That is, the white organic light emitting element(s) may be provided over the first substrate whereas the red, green, and blue organic light emitting elements may be provided over the second substrate.
A structural example in which only the white organic light emitting element(s) is/are provided over one of the first and second substrates in each of the above described examples. Alternatively, any one of red, green, and blue organic light emitting elements may be provided over the substrate having a white organic light emitting element, whereas other two colored organic light emitting elements except for the organic light emitting element provided over the same substrate as the white organic light emitting element, may be provided over the other substrate. That is, for example, red and green organic light emitting elements may be provided over any one of a first substrate and a second substrate, and blue and white organic light emitting elements may be provided over the other substrate.
In a structure in which a white organic light emitting element and any one of red, green, and blue organic light emitting elements are provided over either a first substrate or a second substrate and the remaining two colored organic light emitting elements among the red, green, and blue organic light emitting elements (two colored organic light emitting elements other than the one of the red, green, and blue organic light emitting elements provided over the one of the first and second substrates) are provided over the other substrate, when areas of light emitting regions of the red and blue organic light emitting elements are set larger than an area of a light emitting region of the green organic light emitting element, it is possible to balance the red, green, and blue colors of light. This is because the visibility of the red and blue light is lower than that of the green light.
Further, since the two colored organic light emitting elements are provided over each of the first and second substrates, the number of elements required for driving the organic light emitting elements provided for each dot can be reduced as compared to a case of providing three colored organic light emitting elements over a substrate while having the same number of pixels, and therefore, aperture ratio can be improved.
In the second structure of the present invention, red, green, and blue organic light emitting elements and one to three colored organic light emitting element(s) whose color(s) is/are selected from a complementary color of red, a complementary color of green, and a complementary color of blue, are provided over either a first substrate or a second substrate. That is, one pixel includes four to six colored organic light emitting elements with a combination of the red, green, and blue and one to three colors selected from the complementary colors of red, green, and blue.
As the second structure of the present invention, for example, it is possible to give an example in which red, green, and blue organic light emitting elements are provided over one of a first substrate and a second substrate, whereas one to three colored organic light emitting element(s) selected from the organic light emitting elements of the complementary color of red, the complementary color of green, and the complementary color of blue, is/are provided over the other substrate.
The complementary color of red is blue-green (cyan), the complementary color of blue is yellow, and the complementary color of green is red-purple (magenta). By forming organic light emitting elements having these colors, colors which cannot be expressed by only red, green, and blue can be expressed, thereby widening the range of colors to be expressed and largely improving reproducibility of colors.
Note that, it is not necessary to form all complementary colors of organic light emitting elements with respect to red, green, and blue. The kinds or the number of colors of organic light emitting elements provided over the second substrate can be changed in accordance with quality of an image to be displayed. That is, one to three colors may be selected from the complementary colors of red, green, and blue in accordance with quality to be required.
Although the structural example in which the red, green, and blue organic light emitting elements are provided over either the first substrate or the second substrate, is described above, the present invention is not limited thereto. Alternatively, two colored organic light emitting elements selected from red, green, and blue organic light emitting elements may be provided over one of first and second substrates, and a one colored organic light emitting element, which is a remaining organic light emitting element of the red, green, and blue organic light emitting elements, and a one colored organic light emitting element or two colored organic light emitting elements whose color(s) is/are selected from the complementary colors of red, green, and blue, may be provided over the other substrate.
Since the visibility of red and blue light is lower than that of green light, when areas of light emitting regions of the red and blue organic light emitting elements are set larger than an area of a light emitting region of the green organic light emitting element, it is possible to balance the red, green, and blue colors.
As such a structural example, the following example can be given: a red organic light emitting element is provided over a first substrate; a blue organic light emitting element is provided over a second substrate; a green organic light emitting element is provided over either the first substrate or the second substrate; a one colored organic light emitting element selected from complementary colors of red, green, and blue is provided either the first substrate or the second substrate; and areas of light emitting regions of the red and blue organic light emitting elements are set larger than an area of a light emitting region of the green organic light emitting element.
As compared to a case where three colored organic light emitting elements are provided over a substrate while having the same number of pixels provided over the substrate, when two colored organic light emitting elements are provided over each of the first and second substrates, the number of elements required for driving the organic light emitting elements provided in each dot can be reduced, making it possible to improve aperture ratio.
Further, in the second structure of the present invention, emission colors of organic light emitting elements may be any colors which can widen a range of a triangle of a chromaticity coordinate of red, a chromaticity coordinate of green, and a chromaticity coordinate of blue, in addition to the complementary colors of red, green, and blue. Therefore, an organic light emitting element which emits light having a chromaticity coordinate outside of the triangle of the chromaticity coordinate of red, the chromaticity coordinate of green, and the chromaticity coordinate of blue, may be used. Therefore, one colored to three colored organic light emitting element(s) which emits/emit a color of light having a chromaticity coordinate outside of the triangle of the chromaticity coordinates of red, green, and blue, may be provided as a substitute for the complementary colors of red, green, and blue.
In the third structure of the present invention, red, green, and blue organic light emitting elements are provided over each of a first substrate and a second substrate. Specifically, one pixel includes red, green, and blue organic light emitting elements provided over the first substrate and red, green, and blue organic light emitting elements provided over the second substrate.
In the third structure of the present invention, when the organic light emitting elements provided over the first substrate and the organic light emitting elements provided over the second substrate are overlapped with one another such that the same colored organic light emitting elements are overlapped with one another, display can be performed at more higher luminance. Accordingly, more gray scales can be expressed.
Further, in this case, when light emitting regions of the same colored organic light emitting elements are at least partly overlapped with one another, display can be performed at higher luminance. However, it is preferable that overlapping areas of the different colored organic light emitting elements be increased so as to prevent reduction in aperture ratio. That is, when light emitting regions of the organic light emitting elements provided over the first substrate are overlapped with light emitting regions of the organic light emitting elements provided over the second substrate at almost the same positions as seen from a viewer side, the highest aperture ratio can be obtained.
In the third structure of the present invention, when the light emitting regions of the different colored organic light emitting elements provided over the first and second substrates are overlapped with one another as seen from a viewer side, image resolution performance can be improved.
Note that, in this case, when the light emitting regions of the different colored organic light emitting elements are at least partly overlapped with one another, the image resolution performance can be improved. However, it is preferable that overlapping areas of the different colored organic light emitting elements be increased so as to prevent reduction in aperture ratio. That is, when the light emitting regions of the organic light emitting elements provided over the first substrate are overlapped with the light emitting regions of the organic light emitting elements provided over the second substrate at almost the same positions as seen from a viewer side, the highest aperture ratio can be obtained.
In the fourth structure of the present invention, red, green, and blue organic light emitting elements are provided over a first substrate, and another blue organic light emitting element is provide over a second substrate. Specifically, one pixel includes the red, green, and blue organic light emitting elements provided over the first substrate and the blue organic light emitting element provided over the second substrate.
Since a blue organic light emitting element has low luminance, the luminance of blue emission can be compensated by forming another blue organic light emitting element over the second substrate, making it possible to improve balance of luminance of red, blue, and green.
In the above mentioned fourth structure, a white organic light emitting element may also be provided over the second substrate over which the blue organic light emitting element is provided. By providing the white organic light emitting element over the second substrate, luminance of blue emission can be compensated and brightness of a display screen can be controlled.
In the fourth structure, one or two colored organic light emitting elements selected from complementary colors of red, green, and blue may also be provided over the second substrate over which the blue organic light emitting element is provided. By providing one or two colored organic light emitting elements selected from the complementary colors of red, green, and blue, luminance of blue emission can be compensated and a range of colors to be expressed can be widened.
There are two ways of layout methods of the first substrate and the second substrate of the display device according to the present invention described above.
As a first layout method, a surface of the first substrate over which an organic light emitting element is formed and a surface of the second substrate over which an organic light emitting element is formed are attached to each other so as to face each other.
In accordance with the layout, the second substrate serves as a counter substrate with respect to the first substrate whereas the first substrate serves as a counter substrate with respect to the second substrate. Therefore, the organic light emitting elements can be sealed with only the first and second substrates. Accordingly, the display device can have almost the same thickness as a normal display device in which an organic light emitting element is formed only over one substrate and the substrate is sealed with a counter substrate.
As a second layout method, a surface of the first substrate over which an organic light emitting element is formed is attached to a surface of the second substrate which is opposite to the other surface of the second substrate over which an organic light emitting element is formed.
In the second layout method, the second substrate serves as a counter substrate with respect to the first substrate. Thus, the first substrate can be sealed with the second substrate. However, the second substrate is not sealed with the first substrate. Therefore, a third substrate is provided to face the surface of the second substrate over which the organic light emitting element is formed so as to seal the second substrate with the third substrate.
Accordingly, the organic light emitting elements can be sealed with less number of substrates in the case of the first layout method as compared to the second layout method, so that the first layout method requires lower cost. Moreover, a thickness of a panel formed by attaching the substrates can be more reduced in the first layout method as compared to the second layout method, and hence, the first layout method is more preferable.
Further, as a method for attaching the first and second substrates to seal the organic light emitting elements, various known methods can be employed, for example, the first and second substrates are sealed with a sealant in a sheet-form, a solid sealing material is applied to an entire surface of one of the first and second substrates to be attached, or a sealant is only applied to the circumference of the substrate and a filler is filled between the substrates.
When air does not exist between the first and second substrates by using the method in which a solid sealing material is applied to an entire surface of the substrates to be attached, light extraction efficiency can be improved.
Structures of the first and second layout methods of the first and second substrates will be respectively described in more detail below.
In the first layout method, the following three cases 1, 2, and 3 can be thought. In the case 1, the first and second substrates are light transmitting substrates, and organic light emitting elements provided over the first and second substrates emit light toward both sides of each substrate. In the case 2, the first substrate is a light transmitting substrate and an organic light emitting element provided over the first substrate emits light toward both sides of the first substrate, whereas an organic light emitting element provided over the second substrate emits light toward a surface of the second substrate over which the organic light emitting element is provided, i.e., toward the first substrate. In the case 3, the second substrate is a light transmitting substrate and an organic light emitting element provided over the second substrate emits light toward both sides of the second substrate, whereas an organic light emitting element provided over the first substrate emits light toward a surface of the first substrate over which the organic light emitting element is provided, i.e., toward the second substrate.
In the case 1, display screens are formed over both of the first and second substrates, and an image recognized on the display screen formed over the first substrate is a mirrored image of an image recognized on the display screen formed over the second substrate. Further, since the first and second substrates are the light transmitting substrates, the display screens are formed over the first and second substrates while a viewer can see the view beyond the first and second substrates.
Further, by providing polarizing plates over the first and second substrates, it is possible to prevent a viewer from seeing the view beyond the first and second substrates on both of the display screen formed over the first substrate and the display screen formed over the second substrate.
In the case 2, the display screen is formed only over the first substrate. In the case 3, the display screen is formed only over the second substrate.
Moreover, the following three cases 4, 5, and 6 can be thought in the second layout method. In the case 4, the first and second substrates are light transmitting substrates, and an organic light emitting element provided over the first substrate and an organic light emitting element provided over the second substrate emit light toward both sides of each substrate. In the case 5, the first substrate is a light transmitting substrate and an organic light emitting element provided over the first substrate emits light toward the both sides of the first substrate whereas an organic light emitting element provided over the second substrate emits light toward a surface of the second substrate opposite to the other surface of the second substrate over which the organic light emitting element is provided, i.e., toward the first substrate. In the case 6, the second substrate is a light transmitting substrate and an organic light emitting element provided over the second substrate emits light toward the both sides of the second substrate whereas an organic light emitting element provided over the first substrate emits light toward a surface of the first substrate opposite to the other surface of the first substrate over which the organic light emitting element is provided, i.e., toward the second substrate.
In the case 4, display screens are formed over the first and second substrates, and an image recognized on the display screen formed over the first substrate is a mirrored image of an image recognized on the display screen formed over the second substrate. Further, since the first and second substrates are the light transmitting substrates, the display screens of the first and second substrates are formed while a viewer can see the view beyond the first and second substrates.
Further, by providing polarizing plates over the first and second substrates, it is possible to prevent a viewer from seeing the view beyond the first and second substrates on both of the display screen formed over the first substrate and the display screen formed over the second substrate.
In the case 5, the display screen is formed only over the first substrate. In the case 6, the display screen is formed only over the second substrate.
In the present invention, as a light transmitting substrate, a quartz substrate, a glass substrate, a plastic substrate, or the like is used. When heat treatment at a temperature of 600° C. or more is performed in a process of manufacturing an element such as a thin film transistor or an organic light emitting element over a substrate, a quartz substrate is used. When heat treatment at a temperature of 600° C. or less is performed, a glass substrate or a plastic substrate may be used. Here, the light transmitting substrate indicates a substrate which transmits visible light therethrough. This represents visible light transmittance of the substrate is 80 to 100%.
In each of the above mentioned cases 2 and 5, the light transmitting substrate is used as the first substrate whereas a material of the second substrate is not particularly limited and a substrate having no light transmitting property may be used as the second substrate.
Further, in each of the cases 3 and 6, the light transmitting substrate is used as the second substrate whereas a material of the first substrate is not particularly limited and a substrate having no light transmitting property may be used as the first substrate.
In the above described structures of the present invention, an active matrix display device is preferable.
Organic light emitting elements mentioned in this specification include not only an organic light emitting element having a structure in which a film sandwiched between a first electrode and a second electrode contains an organic compound but also an organic light emitting element having a structure in which a film sandwiched between a first electrode and a second electrode partly contains an inorganic compound in addition to an organic compound.
Organic light emitting materials include a material which can convert energy upon returning to a ground state from a singlet excited state into light emission (a fluorescent material) and a material which can convert energy upon returning to a ground state from a triplet excited state into light emission (a phosphorescent material). Either the fluorescent material or the phosphorescent material may be used as an organic light emitting material used for a display device of the present invention.
As a structure of an organic compound layer, it is possible to employ various known structures such as a structure formed by laminating a hole transporting layer, a light emitting layer, and an electron transporting layer in this order, and a structure formed by laminating a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer in this order.
In a display device of the present invention, a method for driving the display device for image display is not particularly limited, and for example, a dot sequential driving method, a line sequential driving method, a surface sequential driving method, and the like may be used. As a method for expressing a gray scale of an organic light emitting element, either a digital gray scale method or an analog gray scale method may be employed. Further, a source line of the display device may be input with either analog signals or digital signals. A driver circuit and the like may be arbitrarily designed according to image signals.
By providing a substrate having a white organic light emitting element in addition to a substrate having red, green, and blue organic light emitting elements, luminance of only the white organic light emitting element can be increased so that brightness of only a display screen can be improved. Since the substrate having the white organic light emitting element is provided separately from the substrate having the red, green, and blue organic light emitting elements for performing color display, the luminance of the white organic light emitting element can be changed independently from the red, green, and blue organic light emitting elements. Therefore, brightness of the display screen can be easily controlled.
Further, when white color is displayed on a conventional organic EL display which can perform color display, red, green, and blue organic light emitting elements emit light and three colors of light emitted from the organic light emitting elements are mixed to display while color. However, in the structure of the present invention in which a substrate having red, green, and blue organic light emitting elements and a substrate having a white organic light emitting element are overlapped with each other, only the white organic light emitting element emits light while the red, green, and blue organic light emitting elements emit no light so as to display white color. Therefore, power consumption can be more reduced as compared to the case of displaying white color by making the red, green, and blue organic light emitting elements emit light.
When areas of light emitting regions of red and blue organic light emitting elements are set larger than an area of a light emitting region of a green organic light emitting element, it is possible to balance the red, green, and blue colors.
In the case where an organic light emitting element which emits light with a color by which a range of a triangle of a chromaticity coordinate of red, a chromaticity coordinate of green, and a chromaticity coordinate of blue can be widened, is provided separately from red, green, and blue organic light emitting elements, colors which cannot be expressed only by red, green, and blue organic light emitting elements, can be expressed, so that more microscopic and realistic images can be displayed.
Furthermore, when red, green, and blue organic light emitting elements are provided over a first substrate and red, green, and blue organic light emitting elements are provided over a second substrate and the first and second substrates are attached to each other such that the different colored organic light emitting elements provided over the first and second substrates are overlapped with one another as seen from a viewer side, image resolution performance can be improved.
Moreover, when a substrate over which red, green, and blue organic light emitting elements are provided is overlapped with a substrate over which a blue organic light emitting element is provided, luminance of blue light emission can be compensated, making it possible to improve balance of luminance of red, green, and blue.